1. Field of the Invention
The invention pertains to the fiber optics technology, particularly with respect to a phase modulator for modulating the phase of light waves propagating in an optical fiber. The invention has particular application in the fiber optics rotation sensor technology.
2. Description of the Prior Art
An all fiber optical phase modulator with a high operating frequency, wide modulation bandwidth and high modulation efficiency (modulation index) is a desideratum that is not available in the prior art. Existing fiber optical phase modulators, such as the PZT stretcher and PVF.sub.2 coated fiber, have either narrow bandwidth or poor modulation efficiency. The PZT stretcher phase modulator is constructed by winding the fiber around a piezoelectric cylinder. The cylinder stretches the fiber, changing the fiber length when a voltage is applied to the cylinder electrodes. Such devices exhibit low operating frequency and narrow bandwidth. An optical fiber coated with a piezoelectric plastic jacket, such as PVF.sub.2, is also utilized as an optical phase modulator. The piezoelectric jacket squeezes the fiber by applying a voltage across the inner and outer electrodes thereof thereby changing the optical path length and hence modulating the phase. Such devices exhibit low modulation efficiency even though the frequency response thereof is broad. Non-fiber integrated optical devices, such as LiNbO.sub.3 channel waveguide phase modulators, are available in the prior art with both high modulation efficiency and wide bandwidth. When endeavoring to insert such devices into the fiber system, the coupling of light to and from the fiber with respect to the device suffers from high insertion loss and large backscattering which generally results in severe instability in the fiber optic apparatus in which the phase shifter is utilized. Additionally, mechanical and thermal instabilities of such devices and the coupling thereof into the fiber system seriously degrade performance of the fiber optical system in which the phase shifter is utilized.
An all fiber phase modulator having a large modulation bandwidth and index is highly desirable in a fiber optical rotation sensor (gyro) for providing dynamic phase bias and closed-loop operation. Such a phase modulator is required to provide a dynamic bias for maximizing the sensitivity of the rotation sensor when it is utilized to measure very small rates. In the closed-loop configuration, a loop closure transducer, such as a frequency or phase shifter, is located at the end of one fiber to produce a non-reciprocal phase shift. The non-reciprocal phase shift offsets the rotation-induced Sagnac effect phase shift. The closed-loop architecture provides a rotation sensor with wide dynamic range from which a digital output can be obtained.
For both the dynamic phase bias and closed-loop applications, an all fiber optical phase modulator having large modulation bandwidth and index is highly desirable in a fiber optical rotation sensor. When endeavoring to utilize the integrated optical devices, as described above, the coupling of light to and from the fiber with respect to the device suffers from high insertion loss and large backscattering resulting in severe instability in the rotation sensor. Additionally, the mechanical and thermal instabilities described above seriously degrade the operation of the sensor. Thus, for the reasons described above, utilization of the prior art phase shifters in the fiber optical rotation sensor is rendered impractical.
It is appreciated that an all fiber optical phase modulator with large modulation bandwidth and index has very significant applications in fiber optical technologies, and in particular, in fiber optical gyros. The fiber optical gyro has the potential to satisfy the performance requirements for high accuracy applications. The closed-loop fiber optical gyro exhibiting large dynamic range and linear output requires a broadband phase modulator or a pure frequency shifter. It is desirable to utilize an all fiber phase modulator or frequency shifter in the closed-loop gyro for the reasons described above. A wide band fiber optic phase modulator capable of providing a large bandwidth and modulation index is suitable for utilization in a closed-loop fiber optic gyro employing the serrodyne modulation technique to shift the optical frequency. Since an all fiber device can be directly spliced into any fiber optical system, the mechanical and thermal instabilities suffered when utilizing the prior art phase modulators can be eliminated and the rate sensor performance substantially improved.